Why Activated Titanium Anodes are Used for Chlorate Production?

Why Activated Titanium Anodes are Used for Chlorate Production?

Ru02/Ti02-coated titanium electrodes are now a unique anode material used worldwide for caustic-chlorine, chlorate, and seawater electrolysis cells. Million tons per year of the total chlorine production employs catalytically activated titanium anodes.

In addition, there are industrial processes that need even more selective electrocatalysts to provide maximal efficiency. Chlorate production and seawater electrolysis also need activated titanium anodes.

In a chlorate cell, the hypochlorite species undergo a chemical conversion to chlorate which gives maximal current efficiency.Evolution of oxygen by electrolysis of water. The reversible potentials for chlorine and oxygen discharge are close together, so a selective electrocatalyst is needed to suppress the reaction by providing a high oxygen overpotential. In addition, the catalyst must resist corrosion at high anodic current density in solutions containing chloride, chlorine, and hypochlorite.

Oxide catalysts usually produce oxygen more easily than the corresponding bare metal. Ruthenium is one of the less expensive and more available precious metals and its mixed oxide with titanium (Ru02/Ti02) is a highly active catalyst for chlorine evolution while being quite durable. For this reason, it has been widely employed for coating titanium anodes.

Chloride ions inhibit the formation of surface oxides on platinum group metals by being adsorbed onto the surface with the exception of platinum itself which does form surface oxides and thereby exhibits a higher anodic polarization. Among base metals, tin has a remarkable overpotential but corrodes in solutions containing hypochlorites. A palladium tin compound (PdSn2), however, is expected to provide both selectivity (high oxygen overpotential) and a lower corrosion rate.

The ability of transition metal ions to form covalent bond" with a metal surface, leading to high coverages and partial surface blocking. Phosphate ions are known to adsorb onto palladium and thus might contribute to its selectivity as a catalyst. Palladium itself could perhaps be the sole electrocatalyst but RuOdTi02 is cheaper, more active, and more resistant to corrosion. Thus, a composite coating of Ru02/Ti02 and PdSn2 has been optimized as an anionic selective catalyst for chlorate cells and seawater electrolysis.